This invention relates to a pile foundation as structural foundation used in a frigid area and, more particularly, to a frost damage proofed pile or pile secured against frost damage.
When a rack for a pipeline or a similar structure is installed in a frigid area such as permafrost area or seasonally frozen soil area, it is essential to protect the structure against frost damage such as freezing, frost heaving or thawing and resulting ground subsidence. To this end, various construction methods have been resorted to, including using a piling foundation, which appears to be most popular.
By permafrost area is meant a terrain or area such as Alaska, Canada or Siberia where the soild layer frozen through the year, hereafter referred to as permafrost, is distributed and where the annually averaged temperature is lower than 0.degree. C. By the active layer is meant the soil layer extending from the ground surface to the permafrost layer. The active layer is affected severely by seasonal changes in temperature and subjected to freezing and frost heaving during winter while being also subjected to thawing and ground subsidence during summer. By the seasonally frozen soil area is meant the terrain where the annually averaged temperature is higher than 0.degree. C. The seasonally frozen soil area is devoid of the permafrost and subjected to freezing during winter and thawing during summer. In the following description, the seasonally frozen soil layer is used synonymously as the active layer.
The pile foundation used in a frigid area is embedded in the permafrost for supporting the weight of the superstructure and the frost heaving force and the negative friction on the basis of the adfreeze strength between the permafrost and the pile surface. To this end, it is essential to provide for a positive freezing strength between the pile and the permafrost and a sufficient length of pile embedment in the permafrost. However, the permafrost is not necessarily of uniform properties, but may have variable values of freezing strength, depending upon the soil quality or temperature. Thus the structure may actually be subjected to frost damage even if the pile is embedded into the permafrost a distance designed to give a sufficient adfreeze strength. Thus it is required that a certain safety factor be taken into account when designing the pile length, thus causing disadvantages from the viewpoint of ease of construction and economy. There is a great need for an improved technique and system in setting piles in a frigid area.